Mechanism of operation of the asynchronous electric motor
To protozoa on the device and the most widespread the asynchronous engine invented by M.O. Dolivo-Dobrovolsky is. The principle of its work is based on action of the rotating magnetic field on the short-circuited winding adapted for rotation. For strengthening of magnetic field and giving of the due configuration of the winding of the asynchronous engine to it are placed on two cores which gather from sheets of electrotechnical steel 0.5 mm thick. Sheets are from each other isolated by the varnish coat for reduction of losses by whirling currents.
At the motionless part of the machine, the stator, the core has the form of the full cylinder. In grooves from the inside of this core the three-phase winding is laid. This winding joins under tension of three-phase network, and currents arising in it excite the rotating magnetic field of the machine.
At the mobile part, the rotor, the core has the cylinder form. It is strengthened on the machine shaft. In grooves on the surface of the core the rotor winding, in most cases short-circuited is placed. If mentally to remove it from the core, then it will have the appearance of the cylindrical cage from the copper or aluminum rods closed at end faces by two rings from the same material. Such winding is called "the squirrel wheel". Rods of the winding are inserted into rotor grooves without isolation. Often short-circuited winding of the rotor is made by filling by the melted aluminum of grooves of the core. And also the closing rings are cast.
The winding of the stator of the electric motor is carried out by the insulated conductor and keeps within stator grooves. Each of coils is distributed on several grooves. If the winding consists of three coils, then the three-phase system of the currents which are flowing round it excites the above described bipolar rotation. For one alternating-current period such field does one turn. Therefore, with the standard industrial frequency of 50 Hz, i.e. 50 periods a second, the bipolar field does to 50 x 60 =3000 rpm. Rotor rotational speed usually is only several percent less than field rotational speed.
To receive the engine with the smaller speed of the field, it is necessary to increase number of poles of the rotating magnetic field by means of the multipole winding. To each three coils of the stator winding there corresponds one couple of poles of the rotary field. Therefore, if the three-phase winding of the stator consists from To coils. that number of couples of poles of the rotary field excited by this winding will be: É = K:Z.
The rotation sense of the rotor of the asynchronous engine is defined by the rotation sense of its magnetic field.
And the rotation sense of the field is caused by phase sequence And, In, From three-phase network. For veering of rotation of the engine it is enough to change connection of the winding of the stator with network that the stator clip connected originally to the phase A of network would be attached to the phase In network. Respectively, the stator clip connected to the phase In network has to be connected to the phase A of network. Connection of the third clip of the stator with network remains without changes.
While the rotor is not mobile, conditions in the asynchronous engine are similar to conditions in the transformer: to primary winding of the transformer there corresponds the stator winding, and secondary - the rotor winding. Tension on clips of each phase winding of the stator is counterbalanced with the EMF induced in this winding by the rotating magnetic field. Current in the winding of the rotor is induced by the rotating magnetic field.
According to Lenz's principle, this induced current aims to weaken the magnetic field inducing it. But easing of magnetic field reduces the EMF induced by this field in the stator winding. Therefore, electric balance on stator clips is broken. The unbalanced surplus of tension is so formed. It causes increase in current intensity in the stator winding. Current of the stator increases magnetic field approximately up to its former size, and electric balance on clips of the stator is recovered.
The ratio of currents of the stator and rotor in the asynchronous engine is similar to ratios of primary and secondary currents in the transformer. Current of the stator is not magnetizing, and rotor current – demagnetizing. Any change of current of the rotor causes proportional change of current of the stator.
At launch of the engine in the course the rotating magnetic field crosses the rotor winding with the high speed (angular speed of W:P) and induces in it considerable EMF. This EMF creates the big starting current in the short-circuited rotor. Respectively, and in the winding of the stator there is the considerable starting current too. It more working current of the engine approximately time in seven. The starting push of current is characteristic of the asynchronous engine with the short-circuited rotor.
As the speed of the rotor increases. the EMF induced in it decreases, and together with it currents of the rotor and the stator decrease. At the end of launch of not loaded engine the current intensity of the rotor has to be such that the torque developed by the engine covered all its mechanical losses from friction in bearings about air, etc.
If to load already rotating asynchronous engine, then the mechanical braking moment on the engine shaft at first will appear longer than the torque and the rotor will reduce n2 speed/. Respectively, the difference of speeds of n1 – n2 of the field and the rotor will increase, i.e. sliding will increase.
The rotary field will cross the rotor with rather high speed and to induce big EMF in the rotor. Increase of EMF will cause increase in current intensity in the rotor. In proportion to current intensity the torque will increase and will counterbalance the braking loading moment on the engine shaft. At the same time increase in current intensity of the rotor will cause the corresponding increase in current intensity of the stator therefore also consumption of power the engine from network will increase. Thus, with increase in loading on the shaft of the engine sliding, current intensities of the stator and consumption of power the engine from network increases.Top